High-speed optical storage technologies have been sought as there is technical development in information technology fields. Phase-change recording materials may be used as recording materials for rewritable optical storage. Reversible phase changes happen to the phase-change recording materials between crystalline and amorphous phases in response to modulation of laser power or pulse width of irradiated laser pulsed light. Information is optically recorded and/or erased with reversible phase change characteristics of the phase-change recording materials.
In conventional optical storage technologies, irradiated laser light has a pulse width on the order of 10 to several tens of nanoseconds. Recording and erasure techniques which use laser light with pulse width shorter than conventional pulse widths have been studied in order to develop technologies for faster optical storage. For example, Non-Patent Document 1 reports that reversible phase changes in phase-change recording film represented by Ge2Sb2Te5 are achieved by a single pulse which is 30 ps in width.
FIG. 18A is a schematic view of amorphization processes disclosed in Non-Patent Document 1. The amorphization processes are described with reference to FIG. 18A.
FIG. 18A shows a sample 900 which is subjected to the amorphization processes. The sample 900 includes a glass substrate 910, A1 thin film 920, which is 100 nm in thickness, ZnS—SiO2 thin film 930, which is 15 nm in thickness, and crystalline Ge2Sb2Te5 thin film 940, which is 50 nm in thickness. The A1 thin film 920 is formed on the glass substrate 910. The ZnS—SiO2 thin film 930 is formed on the A1 thin film 920. The Ge2Sb2Te5 thin film 940 is formed on the ZnS—SiO2 thin film 930.
FIG. 18A shows single-pulse light SPL1 which is 30 ps in pulse width and irradiated from a laser source (not shown). The single-pulse light SPL1 irradiates the crystalline Ge2Sb2Te5 thin film 940. The central energy density of the single-pulse light SPL1 is 52 mJ/cm2. As a result of the irradiation by the single-pulse light SPL1, the Ge2Sb2Te5 thin film 940 becomes amorphous. The light spot diameter LSD of the single-pulse light SPL1 is 240 μm on the Ge2Sb2Te5 thin film 940.
FIG. 18B is a schematic view of the crystallization processes performed for the sample 900 described with reference to FIG. 18A. The crystallization processes are described with reference to FIG. 18B.
FIG. 18B shows a region TR amorphized by the amorphization processes which are described with reference to FIG. 18A. FIG. 18B shows single-pulse light SPL2 irradiating the region TR. The single-pulse light SPL2 is 30 ps in pulse width and 24 mJ/cm2 in central energy density. As a result of the irradiation by the single-pulse light SPL2, the region TR is crystallized.
In FIG. 18B, the power per single-pulse light which is required to amorphize crystalline materials is expressed by the symbol “Pw”. The power per single pulse of light which is required to crystallize amorphous materials is expressed by the symbol “Pe”. According to Non-Patent Document 1, phase-change processes for the Ge2Sb2Te5 thin film 940 are performed under a relationship “Pw>Pe”.
Non-Patent Document 2 reports reversible phase changes of Ge0.07Sb0.93 phase-change recording film. According to Non-Patent Document 2, phase changes are achieved by single-pulse light which is 30 ps in pulse width, like Non-Patent Document 1. Like Non-Patent Document 1, there is the relationship “Pw>Pe” between the power “Pw” and the power “Pe”.
Patent Document 1 discloses techniques to change an amorphized mark into a crystallized mark under irradiation with DC light. The techniques of Patent Document 1 do not aim to selectively cause amorphization and crystallization under irradiation by single-pulse light for overwriting.
The aforementioned Non-Patent Document 1 shows that the single-pulse light used for recording and erasure is 240 μm in spot diameter whereas Non-Patent Document 2 does not show any light spot diameter. However, Non-Patent Document 2 shows that the major axis of the region which is subjected to phase change is no shorter than 100 μm.
According to the various aforementioned documents, a region of a phase-change recording film which is heated by single-pulse light is greater than current optical storages. The phase-change recording films disclosed in Non-Patent Documents 1 and 2 are no thinner than 25 nm, which is thicker than current optical storages.
The inventors used small light spot diameters in various experiments to perform recording and erasure processes for thin phase-change recording films, in order to perform recording and erasure for very high-density recording media using single-pulse light. As a result of the various experiments, the inventors figured out that conventional technologies face difficulties in crystallization of thin phase-change recording films.    Patent Document 1: JP H6-131663 A    Non-Patent Document 1: J. Siegel et al, “Rewritable phase-change optical recording in Ge2Sb2Te5 films induced by picosecond laser pulses”, Appl. Phys. Lett., Vol. 84, 2250-2252    Non-Patent Document 2: J. Siegel et al, “Dynamics of ultrafast reversible phase transitions in GeSb films triggered by picosecond laser pulses”, Appl. Phys. Lett., Vol. 75, 3102-3104